Variable displacement hydraulic pumps include a number of controls for protecting pumping systems and pump components and for reducing wasted pump power during operation. For example, pressure compensator controls reduce displacement when outlet pressure becomes too high and load sense controls make adjustments to change in load pressure to improve efficiency.
An additional problem which presents difficulties to variable displacement hydraulic pumps is cavitation which can destroy a variable displacement pump or severely limit its life. Cavitation generally occurs when the fluid supply pressure on the inlet or suction side of a variable displacement pump is too low (high vacuum). The hydraulic fluid vaporizes in a vacuum which results in gas bubbles in the oil. The bubbles are carried through the pump and collapse with considerable force when exposed to the load pressures at the outlet. In addition to adversely affecting the pump itself, gas bubbles can damage the consuming device receiving hydraulic fluid from the pump.
One approach to this problem is to avoid cavitation by increasing the flow of input fluid upon cavitation being detected so that air is not drawn into the pump and system. With this approach, the pump tends to exceed its designed capacity upon experiencing cavitation. It is, of course, not prudent to exceed the design capacity of a device and it is usually not economically desirable to over design a device so as to accommodate cavitation, if such over design can be avoided. In any event, there are difficulties with such an approach.
The low inlet pressure and resulting vacuum which creates cavitation, can be due to a number of conditions such as restricted inlet area, high viscosity oil, a clogged inlet filter or a clogged strainer. At least with respect to variable displacement pumps in which the pistons move in an axial direction, no attempt has been made to prevent cavitation by decreasing displacement. By so doing, the risk of operating a pump at over capacity, or the disadvantages, both economic and technical, of overdesigning pumps can be avoided.